1. Field of the Invention
The invention relates to methods for manufacturing semiconductor devices and other finely structured components, and to projection exposure systems for carrying out the methods.
2. Description of the Related Art
Photolithographic projection objectives have been used for several decades to manufacture semiconductor devices and other finely structured components. They serve the purpose of projecting patterns of photomasks or lined plates, also referred to below as masks or reticles, onto a photosensitive substrate, for example a semiconductor wafer coated with a photosensitive layer, with very high resolution on a demagnifying scale.
A number of approaches are followed in order to produce ever finer structures of the order of magnitude of 100 nm or therebelow. Firstly, an attempt is made to increase the image-side numerical aperture (NA) of the projection objective beyond the currently achievable values in the range of NA=0.8 or above. Again, ever shorter operating wavelengths of ultraviolet light are being used, preferably wavelengths of less than 260 nm, for example 248 nm, 193 nm, 157 nm or therebelow. Finally, yet other measures are being used to increase resolution, for example phase-shifting masks and/or oblique illumination.
The shortening of the operating wavelength λ in the region below 193 nm is rendered difficult by virtue of the fact that only a few sufficiently transparent materials, in particular fluoride crystals such as calcium fluoride or barium fluoride, are available for manufacturing lenses for this wavelength region. These materials are available only to a limited extent and present a problem with regard to their birefringent properties at 193 nm and, in particular, at 157 nm.
When the aperture is increased substantially above NA=0.85, limits are reached for the angle load, especially for lenses near the image. Larger apertures than NA=0.95 as far as NA=1 are regarded as infeasible. At apertures of NA>1, the edge and coma rays can no longer be coupled out of an objective, because of total reflection.
The use of immersion fluids between the projection objective and substrate can theoretically be used for the purpose of implementing numerical apertures NA greater than 1. However, practical systems for immersion lithography have not yet been published.